why does c_strlen() return a signed integer?

[Martin Sebor <msebor at gmail dot com>]

PR 87059 - internal compiler error: in set_value_range, at
tree-vrp.c:289, is apparently due to an argument type mismatch
in a MIN_EXPR introduced by expand_builtin_strncmp().

The function calls c_strlen() to compute the length of the two
string arguments to strncmp() and then replaces the third (bound)
argument with a MIN (len, bound).  The type mismatch is caused by
len being the signed result of c_strlen(), while bound being
the size_t argument (explicitly converted to sizetype by
the function).

My question is: why does c_strlen() return a ssizetype, a signed
type, rather the more natural sizetype?  I ask in part because
changing expand_builtin_strncmp() to convert the bound to
ssizetype has downstream effects in the test suite, in part
because it seems unexpected for the function to return a signed
type given that the result cannot be negative, and in part also
because I see most callers explicitly convert the result to
size_type_node.

Thanks
Martin